This invention relates to a process for treating metal surfaces and a treating solution for use in such a process. The invention also relates to a metal surface treated by the process of the invention. The process is particularly useful for cleaning metal surfaces, such as in a pretreatment of metal surfaces. In such a pretreatment application, the process may provide a uniform and chemically active surface prior to further surface treatment, such as the application of a coating by painting, conversion coating, anodising or plating.
In technologies dealing with pretreatment of metal surfaces, a clean uniform metal surface is often crucial in the overall effectiveness of the treatment process. In particular, a uniform, chemically active metal surface is very important for the adherence of an applied coating such as paint, powder coatings, polymer coatings and conversion coatings.
While surface impurities and/or contamination can be successfully removed by mechanical abrasion of the metal, mechanical abrasion is labor intensive and therefore uneconomical. It may also lead to excessive pitting and other damage to the surface. Chemical cleaning is therefore generally favoured.
One common means of chemically cleaning metal surfaces is by treatment with alkaline based solutions. Such solutions dissolve contaminants and impurities such as oxides from the surface of the metal, but may also etch surface oxides and/or metal. The result is often that a smut is left on the surface of the metal which requires further treatment of the metal to remove it. As used herein, the term xe2x80x9csmutxe2x80x9d is intended to include impurities, oxides and any loosely-bound intermetallic particles which as a result of the alkaline treatment are no longer incorporated into the matrix of the alloy.
Traditionally, removal of smut left after alkaline treatment has been effected by acidic solutions having effective amounts of appropriate additives. These xe2x80x9cde-smuttingxe2x80x9d, or xe2x80x9cdeoxidisingxe2x80x9d, solutions remove smut from the metal surface and preferably etch the metal surface to remove oxide scale in order to leave a substantially homogeneous surface for any subsequent treatment. Many such prior desmutting solutions contain chromium ions. The use of chromium-containing desmutting solutions is particularly prevalent, but not restricted to, the field of metal conversion coatings. The term xe2x80x9cconversion coatingxe2x80x9d is a well known term of the art and refers to the replacement of native oxide on the surface of a metal by a controlled chemical formation of a chemical film. Oxides or phosphates are common conversion coatings. Conversion coatings are used on metals, such as aluminium, steel, zinc, cadmium or magnesium and their alloys, and provide a key for paint adhesion and/or corrosion protection of the substrate metal. Accordingly, conversion coatings find application in such areas as the aerospace, architectural and building industries.
In recent years however it has been recognised that the hexavalent chromium ion, Cr6+, is a serious environmental and health hazard. Consequently, strict restrictions have been placed on the quantity of Cr6+ used in a number of industrial processes and limitations placed on its release to the environment, leading to costly effluent processing.
There is clearly a need for an alternative metal treating solution which effectively cleans metal surfaces but does not pose the same environmental and health risks of the prior art.
An object of the present invention is therefore to overcome, or at least alleviate, one or more of the difficulties and/or deficiencies related to the prior art.
Accordingly, the present invention provides a process for cleaning a metal surface including the steps of:
(a) contacting said metal surface with an alkaline cleaning solution in order to remove contaminants such as dirt and grease; and
(b) contacting said metal surface with an acidic, rare earth ion containing solution thereby to remove smut formed on said metal surface by step (a).
The present invention also provides an acidic, rare earth ion containing aqueous cleaning solution for use in step (b) of the process defined in the preceding paragraph, said solution including ions of one or more rare earth ions, wherein the pH and concentration of rare earth ions in solution are effective to remove smut from a metal surface previously contacted with an alkaline cleaning solution.
Steps (a) and (b) of the treating process of the present invention may be used as a pretreatment of a metal surface prior to a subsequent finishing treatment such as applying paint or a coating. It is particularly useful as a pretreatment of metal surfaces prior to the application of a conversion coating thereto, such as a rare earth element based conversion coating.
One such conversion coating process has been described in Australian patent specification AU-A-14858/88. The conversion coating process comprises contacting a metal surface with a solution formed by an aqueous acidic solution containing cerium cations and H2O2 in which some or all of the cerium cations have been oxidised to the +4 valence state. Gaseous evolution in the region of the metal surface causes an increase of the solution pH to a sufficiently high value to precipitate a cerium containing coating on the metal surface.
Accordingly the present invention further provides a process for forming a rare earth element containing coating on the surface of a metal, including the steps of:
(a) contacting said metal surface with an alkaline cleaning solution to remove surface contaminants such as dirt, grease and oxides;
(b) contacting said metal surface with an acidic, rare earth ion containing cleaning solution thereby to remove smut formed on said metal surface during step (a); and
(c) contacting the metal surface with an aqueous acidic, rare earth ion containing coating solution including rare earth cations capable of having more than one valence state, resulting in an increase of the pH of the acidic solution in the region of the metal surface to a value sufficient to precipitate one or more compounds of the rare earth element, thereby to cause the compound of the rare earth element to precipitate in a coating on the metal surface.
Pretreatment of the metal surface by steps (a) and (b) of the present invention is found to result in improved corrosion resistance and/or at least similar adhesion characteristics of the subsequently applied coating compared to the properties of a rare earth element based coating applied to a metal surface which was not subjected to any pretreatment or was instead pretreated with a chromate based cleaning solution. Also, the rare earth pretreatment results in a shorter time being subsequently required to deposit the rare earth element-based coating, as compared to other metal pretreatments, such as Cr based deoxidising solutions. Moreover, the absence of Cr6+ in the solutions used significantly reduces the risk to health and the environment.
The step of contacting with an alkaline cleaning solution may be preceded by a degreasing step in which the metal surface is contacted with a degreasing composition, such as trichloroethane or a solution available under the trade name of BRULIN, which is an aqueous degreasing solution. A degreasing step may be necessary, for example, where the metal has been previously coated with lanoline or other oils or grease or with a plastic coating.
The alkaline cleaning solution is preferably a xe2x80x9cnon-etchxe2x80x9d solution, that is, one for which the rate of etching of material from the metal surface is slow. A suitable alkaline cleaning solution is that commercially available under the trade name RIDOLINE 53.
The treatment with an alkaline cleaning solution is preferably conducted at an elevated temperature, such as up to 80xc2x0 C., preferably up to 70xc2x0 C.
Preferably the metal surface is rinsed with water between each of the above steps (a) to (c).
Treatment with the acidic, rare earth ion containing cleaning solution of step (b) is designed to remove smut left on the metal surface after step (a). The acidic, rare earth ion containing solution preferably comprises at least one rare earth compound dissolved in a mineral acid solution. The mineral acid may be sulphuric acid or nitric acid or a mixture of mineral acids such as sulphuric acid and nitric acid. However, preferably, the mineral acid is sulphuric acid. The rare earth ion solution. must be sufficiently acidic to assist in the removal of the smut on the metal surface. In most instances, this will necessitate a pH of less than 1, preferably less than 0.5.
Preferably the rare earth ion in the acidic, rare earth ion containing cleaning solution should possess more than one higher valence state. By xe2x80x9chigher valence statexe2x80x9d is meant a valence state above zero valency. Without wishing to be limited to one particular mechanism of smut removal, it is believed that the multiple valence states of the rare earth ion imparts a redox function enabling the rare earth ion to oxidise surface impurities and result in their removal as ions into solution. Such rare earth ions include cerium, praseodymium, neodymium, samarium, europium, terbium and ytterbium ions. The preferred rare earth ions are cerium ions and/or a mixture of rare earth ions. Preferably, the rare earth compound is cerium (IV) hydroxide, cerium (IV) sulphate, or ammonium cerium (IV) sulphate, while the mineral acid preferably is sulphuric acid.
The rare earth compound is present in the cleaning solution in an effective quantity and may be present in solution in a concentration up to saturation of the rare earth compound. Throughout the specification, values of concentration of rare earth ion in solution are mainly expressed as the equivalent grams of cerium per liter of solution. The acidic, rare earth ion containing cleaning solution may have in excess of 0.001 grams of the rare earth ion per liter of mineral acid solution. In some applications, the rare earth ion may be 10 ppm or above. The cleaning solution may furthermore have in excess of 0.01 grams, such as in excess of 0.014 grams per liter. However, for most applications of the invention, the cleaning solution has a concentration of rare earth ions of at least 0.1 g/l, such as 0.7 g/l (0.005M) or higher. It is preferred, however, that the minimum concentration of rare earth ions in the cleaning solution is 7.0 g/l (0.05M) and a concentration of at least 10 g/l may therefore be appropriate. The upper concentration limit of the rare earth ion in the cleaning solution is normally around 100 grams per liter, although in some embodiments, the concentration can be as high as 140 g/l (1M). However, there may be little cost benefit at such high concentrations. Usually concentrations of 80 g/l or below are more appropriate. Preferably, there is less than 70 grams, more preferably less than 50 grams, of the rare earth ion per liter of said solution. Preferably, the amount of rare earth ion does not exceed 30 grams per liter of solution. The concentration may advantageously be less than 21 grams/liter, such as less than 20 grams/liter. A suitable concentration for some applications is below 18 grams/liter such as less than 16 grams/liter. For these applications it is further preferred that the concentration be below 15 grams/liter, such as around 14 grams/liter and below.
The total concentration of mineral acid in the rare earth ion containing cleaning solution is preferably below 5 molar, such as below 4 molar. More preferably, however, the mineral acid has a concentration of up to 3 molar. For most applications, the mineral acid concentration is below 2.75 molar and in some embodiments it is 2.5M or lower. The lower concentration limit of the mineral acid may be 0.5 molar although under some conditions it can be as low as 0.1M. In some embodiments, the lower limit is preferably 1 molar. In preferred embodiments, a suitable concentration of mineral acid is above 1.7 molar such as up to about 2 molar.
If desired, the cleaning solution may optionally include one or more etch rate accelerators which increase the rate of etching of the metal surface. Inclusion of one or more of these etch rate accelerators in the cleaning solution may increase the rate of deposition of the subsequently applied conversion coating. Moreover, including one or more of these etch rate accelerators in the cleaning solution may lead to greater adhesion of a subsequently applied coating, in particular a conversion coating.
The etch rate accelerator may comprise one or more of the following species: halide ions, phosphate ions, nitrate ions and titanium ions. Of the halide ions, fluoride and/or chloride ions are preferred.
Fluoride ions may be added to the acidic, rare earth ion containing cleaning solution in the form of HF or, preferably, as ammonium bifluoride (NH4F.HF) or potassium bifluoride (KF.HF). The preferred concentration of Fxe2x88x92 is less than 0.3M, such as up to approximately 0.2M. A suitable upper concentration is 0.15M. The lower limit of Fxe2x88x92 concentration may be 0.01M. In some embodiments, the lower limit of Fxe2x88x92 concentration is 0.015M. In a preferred embodiment, the concentration of Fxe2x88x92 is around 0.05M. The maximum preferred amount of Fxe2x88x92 in solution depends on whether HNO3 is also present, as higher Fxe2x88x92 concentrations can exist with HNO3 also present in solution.
Phosphate ions are preferably added to the rare earth ion containing cleaning solution as H3PO4. A preferred upper limit of phosphate concentration is 0.05M although for most applications 0.015M is a sufficient upper limit. The lower limit of phosphate concentration may be around 0.001M. However, preferably the phosphate ions are present in the cleaning solution at a concentration of 0.01M or higher, such as around 0.015M.
If desired, the cleaning solution may also include nitrate ions, preferably added in the form of HNO3. HNO3 may be present in the cleaning solution at a concentration of up to 160 g/l. However, for some embodiments of the invention a preferred concentration is around 80 g/l or below. In other embodiments, the concentration of nitrate ions is less than 50 g/l, such as less than 40 g/l. In another embodiment, the upper limit is around 10 g/l. The lower limit of HNO3 concentration may be 1 g/l. In one embodiment, the HNO3 concentration is around 3.15 g/l (0.05M).
If Ti ions and/or Cl ions are to be added to the cleaning solution, they are preferably added as TiCl4. Another source of Ti ions is fluorotitanic acid, (H2TiF6). Titanium ions may be present up to 1000 mg/l. However, preferably Ti ions are present in solution at a concentration below 500 ppm (0.5 g/l), such as 300 ppm (0.3 g/l) or below. In some embodiments, the lower limit of Ti4+ concentration may be around 10 mg/l. In a preferred embodiment, the concentration of Ti ions is 145ppm (0.145 g/l).
If the rare earth ion containing cleaning solution includes as an etch rate accelerator chloride ions, they are preferably present in solution up to a concentration of 0.01 molar, such as up to 0.006 molar. Where chloride ions are added in the form of TiCl4, the amount of chloride ions in solution is preferably the stoichiometric equivalent of the preferred concentration of Ti ions, that is, four times the molarity.
As previously described, the rare earth ion containing cleaning solution preferably comprises a rare earth compound dissolved in a mineral acid solution. If the cleaning solution includes one or more etch rate accelerators which are mineral acids themselves (such as HF, H3PO4, HNO3), the cleaning solution effectively comprises a rare earth compound dissolved in a mixture of two (or more) mineral acids. In such a solution, the total concentration of mineral acid is preferably no greater than 5 molar.
Under some circumstances, the rare earth ion containing solution may beneficially contain additional oxidising agent, such as peroxide or persulphate, in order to assist in the oxidation and removal of smut into solution.
The rare earth ion containing cleaning solution is used at a temperature less than 100xc2x0 C., such as below 85xc2x0 C., preferably below 80xc2x0 C. In some applications, the temperature may be below 70xc2x0 C., and for those applications, the preferred maximum temperature is from 50 to 60xc2x0 C. Preferably, the rare earth ion containing cleaning solution has a temperature of 45xc2x0 C. or lower and, more preferably, the temperature is around 35xc2x0 C. However, the solution may also be used at temperatures around ambient temperature such as from 10 to 30xc2x0 C.
The metal is treated with the acidic, rare earth ion-containing cleaning solution for a period of time sufficient to remove surface smut to the desired degree. Preferably the metal is treated for less than 1 hour, such as up to 50 minutes. In some embodiments, the metal may be cleaned for up to 45 mins such as 30 mins or below. In other applications, the metal is cleaned for up to 20 mins, such as for a maximum of 15 mins. The lower time limit may be as short as about 1 second or it may be longer, such as 5 mins. Alternatively, the minimum period of time may be around 10 minutes.
The etch rate of the rare earth element containing cleaning solution varies according to the composition of the metal or metal alloy. In general, the etch rate can be increased by increasing the temperature of the cleaning solution. Also, as previously discussed, additives such as fluoride ion and/or HNO3 may increase the rate of etching of the metal surface by the rare earth element containing cleaning solution.
The rare earth ion containing coating solution of step (c) also contains at least one rare earth ion having variable valence. Again, the preferred rare earth ion is cerium and/or a mixture of rare earth ions. It is particularly preferred that the rare earth ion be introduced into solution in the form of a soluble salt, such as cerium (III) chloride. However other suitable salts include cerium (IV) sulphate or cerium (III) nitrate. It is further preferred that the cerium be present in solution as Ce3+ cations. Accordingly, when the metal surface is reacted with the coating solution, the resulting pH increase at the metal surface indirectly results in a precipitation of a Ce IV compound on the metal surface. However, the cerium can be present in the solution as Ce4+, if required.
The rare earth ion may be present in the coating solution at a concentration below 50 grams/liter, such as below 40 g/l. Preferably, the rare earth ion is present at a concentration up to 38 g/l. More preferably, the rare earth ion concentration is below 10 g/l, such as below 5 g/l, preferably below 4 g/l. A suitable concentration is 3.8 g/l and below. The lower concentration limit may be 0.038 g/l, such as 0.38 g/l and above.
The coating solution may also contain an oxidising agent. The oxidising agent, if present, is preferably a strong oxidant, such as hydrogen peroxide. It may be present in solution in a concentration up to the maximum commercially available concentration (usually around 30 volume %). Alternatively, the H2O2 may have a maximum concentration of 9 volume %. In some embodiments, the H2O2 concentration is below 7.5%, preferably below 6%, more preferably below 3%. Advantageously, the H2O2 content is low, such as below 1%, preferably below 0.9%, for example about 0.3%. The H2O2 concentration is preferably above 0.03%, such as above 0.15%.
The coating solution may also include a surfactant, in an effective amount, in order to lower the surface tension of the solution and facilitate wetting of the metal surface. The surfactant may be cationic or anionic. Inclusion of a surfactant is beneficial in that by reducing surface tension of the coating solution, it thereby minimises xe2x80x9cdrag-outxe2x80x9d from the solution. xe2x80x9cDrag-outxe2x80x9d is an excess portion of coating solution which adheres to the metal and is removed from solution with the metal and subsequently lost. Accordingly, there is less waste and costs are minimised by adding surfactant to the coating solution. The surfactant may be present in solution at a concentration up to 0.01%, such as 0.005%. A suitable concentration may be up to 0.0025%.
The pH of the coating solution is acidic and may be below 4, such as below 3.0, preferably below 2.8. Advantageously the pH is adjusted to a value below 2.5, such as 2.0 or below, prior to the addition of the oxidant. The lower limit of solution pH may be 0.5 and is preferably about 1.0, such as above 1.5.
The coating solution is used at a solution temperature below the boiling temperature of the solution. The solution temperature may be below 100xc2x0 C., such as below 95xc2x0 C., preferably up to 75xc2x0 C., more preferably up to 50xc2x0 C. The lower temperature limit is preferably ambient temperature.
The metal surface is contacted with the coating solution for a period of time sufficient to give a desired coating thickness. A suitable coating thickness is up to 1 xcexcm, such as less than 0.8 xcexcm, preferably less than 0.5 xcexcm. Preferably, the coating thickness is the range 0.1 to 0.2 xcexcm.
The cleaning and coating steps may be followed by a sealing step. Preferably, the coated metal surface is rinsed prior to and after the sealing process. The rare earth coating may be sealed by treatment with one of a variety of aqueous or non-aqueous inorganic, organic or mixed sealing solutions. The sealing solution forms a surface layer on the rare earth coating and may further enhance the corrosion resistance of the rare earth coating. Preferably the coating is sealed by an alkali metal silicate solution, such as a potassium silicate solution. An example of a potassium silicate solution which may be used is that commercially available under the trade name xe2x80x9cPQ Kasil #2236xe2x80x9d. Alternatively, the alkali metal sealing solution may be sodium based, such as a mixture of sodium silicate and sodium orthophosphate. The concentration of the alkali metal silicate is preferably below 20%, such as below 15%, more preferably 10% or below. The lower concentration limit of the alkali metal silicate may be 0.001%, such as above 0.01%, preferably above 0.05%.
The temperature of the sealing solution may be up to 100xc2x0 C., such as up to 95xc2x0 C., preferably up to 90xc2x0 C. more preferably below 85xc2x0 C., such as up to 70xc2x0 C. The lower limit of the temperature is preferably ambient temperature, such as from 10xc2x0 C. to 30xc2x0 C.
The coating is treated with the sealing solution for a period of time sufficient to produce the desired degree of sealing. A suitable time period may be up to 30 minutes, such as up to 15 minutes, and preferably is up to 10 minutes. The minimum period of time may be 2 minutes.
The silicate sealing has the effect of providing an external layer on the rare earth element coating.